1. Field Of The Invention
This invention relates to a coating composition which provides improved resistance to environmental attack of metallic substrates, by incorporation of metal-coated microspheres which are concentrated at the exposed surface of a dry coating to form an electrically non-conductive barrier, in a metal powder-organic binder composition. The invention has particular utility in the form of a zinc-rich composition containing zinc coated microspheres for deposition on ferrous metal substrates.
2. The Prior Art
Various compositions have been used as protective coatings containing polymers such as phenolic resins, polyesters, polyurethanes, epoxy resins and polyvinyl chloride resins, which also contain metallic chromates or phosphates, metallic oxides and/or zinc in particulate form. Zinc-rich primers have been considered to be optimum anti-corrosion coatings on iron or steel substrates. However, problems have restricted their use as industrial type primers. The action of zinc powder in inhibiting rust is based on an electrochemical interaction between the zinc and the steel substrate. In order not to insulate the zinc particles from each other and from the substrate the prior art has considered it necessary to use very little binder, with satisfactory corrosion protection achieved only when the zinc:binder ratio is at least about 92:8. The high zinc level and the relatively high density of zinc powder often cause undesirable settling during short term storage. Hence, the zinc powder is often added just prior to application and mixed rapidly during application to prevent settling and clogging of spray equipment. This deters efficient field use.
A low binder content was previously considered to be an advantage since a higher binder level would isolate the metallic grains from each other and from the substrate to be protected, thereby providing ineffective cathodic protection.
A lower zinc content is disclosed in U.S. Pat. No. 3,998,771, issued December, 1976 to T. J. Feneis, Jr. et al, which describes water-based coating compositions for application on iron supports to obtain corrosive coatings. Single phase compositions in this patent include about 2% to 10% by weight of a non-volatile liquid epoxy resin, with low viscosity, derived from bisphenol A and an epihalohydrin, e.g., epichlorohydrin; about 2% to 10% by weight of a modified polyamide, i.e., an addition product of a water soluble polyamide and a liquid epoxy resin; and about 55% to 70% by weight of a zinc-powder pigment having an average particle size of about 2 to 15 microns.
U.S. Pat. No. 4,417,007, issued November, 1983 to G. A. Salensky et al, discloses a one component composition containing from about 4% to 25% by weight epoxy or phenoxy resin binder and a polyamine hardener, about 43% to 90% by weight zinc dust, about 3% to 38% by weight Mn.sub.3 O.sub.4 fume pigment, up to 35% by weight additional pigments including pigment extenders and fillers (such as talc, clays, diatomaceous silica and silica), up to 5% by weight pigment suspension agent (such as hydrous magnesium silicate and lecithin), and balance organic solvents. A 1:1 volume ratio of zinc dust:Mn.sub.3 O.sub.4 is preferred.
U.S. Pat. No. 4,891,394, issued January, 1990 to Ronald R. Savin, discloses a coating composition for the protection of metallic and non-metallic substrates against environmental attack, comprising about 10% to about 25% by weight of a film-forming polymer which may be epoxy resins having an epoxide value of about 250 to 2500, vinyl chloride resins copolymerized with polyisocyanates, and/or vinyl chloride resins copolymerized with melamines; about 30% to about 60% by weight particulate metallic zinc; an agent for control of electrical conductive characteristics comprising a crystalline silica having an oil absorption value of less than 20 measured by ASTM Test D281-84, the volumetric ratio of such agent to the metallic zinc ranging from about 0.7:1 to about 1.25:1; about 2% to about 3% by weight of an agent for control of rheological characteristics comprising a pyrogenic amorphous silica having an average particle size less than about 0.012 micron; and at least one solvent compatible with the polymer.
French patent application 2,602,239, published Feb. 19, 1988 in the name of Ronald R. Savin, discloses a two phase coating composition containing up to 70% by weight of a powdered metal (based on the total weight of the composition after admixture), from about 2% to 30% of a film-forming polymer, about 2% to about 30% of a hardener for the polymer, at least 1.8% to 30% of an agent for control of rheological characteristics, and up to 30% by weight organic solvents. The preferred polymer is an epoxy resin having an average molecular weight of 350 to 3800. The agent for control of rheological characteristics comprises at least one pyrogenic silica and optionally at least one natural or transformation silica having an oil absorption value preferably not greater than 90 and more preferably not greater than 40. In the specific examples, pyrogenic silicas were used having average particle sizes of about 0.014 micron, about 0.007 micron and about 0.008 micron, respectively.
U.S. Pat. No. 4,748,194, issued May, 1987 to Geeck, discloses a coating composition for the protection of gas tanks, comprising a powder metal (such as zinc, cadmium, stainless steel, aluminum, alloys or mixtures thereof), a linear epoxy or phenoxy resin having a molecular weight of less than 15,000 cross-linked with a blocked isocyanate, a suspension agent, a thixotropic agent, and "active" and "inactive" organic solvents. The powdered metal ranges from 13 to 52 parts per hundred. The solids content of these compositions is far below federal and state regulations for volatile organic content. The suspension agent disclosed in this patent is polyethylene, and the thixotropic agent is silane treated silica dioxide, in amounts up to 2 parts per hundred.
U.S. Pat. No. 4,621,024, issued Nov. 4, 1986 to F. A. Wright, discloses metal coated microspheres and a process for preparation thereof. Particulate zinc, aluminum, silver, copper, stainless steel, platinum, gold, or mixtures thereof, having an average particle size of about 6 to 10 microns, are bonded to the surfaces of non-conductive microspheres by means of a thermosetting adhesive coating on the microspheres with application of heat, followed by intermittent mixing in the absence of heat. The microspheres may be fly ash, comprising about 80%-96% by weight alumina-silica, with minor amounts of iron oxide, alkaline earth metal oxides and alkali metal oxides The adhesive binder preferably comprises an organo functional silane and a copolymerizable monomer. In the final product the metal is from about 15% to about 30% by weight, relative to the weight of the adhesive binder-coated microspheres. Although this patent discloses average particle size diameters of metal coated microspheres ranging from about 60 to 180 microns, the assignee also produces zinc coated microspheres of smaller average diameters, e.g., about 2.5 to about 60 microns.
The use of zinc-coated microspheres disclosed in the above mentioned U.S. Pat. No. 4,621,024 in zinc-rich inorganic binder compositions has been proposed by the prior art, as a partial replacement for zinc dust. More specifically, substitution of 20% and 40% by volume of zinc-coated microspheres in place of zinc dust in a silicate primer (produced by Carboline Company of St. Louis, Mo., under the trademark "Carbo Zinc 11") has been evaluated. Silicate binders of this type have a very slow drying time, and also require blast cleaning of the metal substrate prior to deposition of the coating. Moreover, such coatings are electrically conductive. Hence such inorganic binder compositions are unsuitable for many industrial applications, such as moving production lines and applying coatings in the field under ambient climatic conditions.
While the above-mentioned patents have provided coatings for metallic or non-metallic substrates which have substantially improved the useful life of the substrates, it has been found that even the best coatings proposed in the prior art exhibit disadvantages which are manifested by blisters, separation from the substrate to be protected, poor adhesion and/or premature corrosion.
It is an object of the present invention to provide coating compositions for the protection of metallic substrates which avoid the disadvantages of prior art compositions as outlined above.
A further object is the provision of a zinc-based coating composition of relatively low density or specific gravity which can be readily applied over untreated metal substrates by means of all conventional application equipment while at the same time meeting all current environmental regulations regarding solvent emissions for volatile organic compounds.
Still another object of the invention is the provision of an electrically non-conductive coating for a ferrous metal substrate providing cathodic protection to the substrate, wherein zinc-coated microspheres are concentrated at the exposed surface of the coating while powdered zinc is concentrated adjacent the substrate, with a polymeric film providing substantially complete impermeability against the penetration of moisture, water vapor or saltspray vapor.